A data communication network includes a communication medium, such as a transmission cable, and a plurality of using devices, such as processors, mass storage devices, or input and output units, connected to the medium for communication.
The interconnection of the devices and the communication medium is commonly achieved by extending the communication medium to all of the devices. If the using devices are located at physically dispersed locations, the medium must be long enough to extend between all of the locations.
This interconnection approach is simple, but has inherent disadvantages stemming from the relatively long length necessitated for the communication medium. These disadvantages relate to the deterioration of electrical characteristics of the medium with increases in the medium's length, as well as to the deterioration of performance characteristics of the network which result from the time delay of signal propagation from one end of the network to the other.
These disadvantages are well exemplified by the linear topology, carrier sense multiple access with collision detection (CSMA/CD) protocol networks that are coming into increasingly greater use. These networks commonly utilize a coaxial cable as the communication medium, and the using units are distributed along the length of the coax and contend with each other for access thereto. A using unit attempts a transmission on the medium only when it detects the medium to be quiescent, and attempts retransmission after it detects a collision, i.e., interference, between its attempted transmission and that of another using unit.
In such a network, the characteristics of a signal transmitted over the medium deteriorate due to factors such as signal attenuation, signal dispersion, and signal noise. The deterioration increases with length of the medium and affects adversely the reliability of communications across the medium. Attempts to improve these characteristics by the use of very high quality, and hence very high cost, coax cable add greatly to system cost. Furthermore, higher quality coax cable generally has a larger diameter, which lowers the flexibility of the cable and thus adversely affects routing capability of the cable.
Coax cable bandwidth decreases with increases in cable length and therefore increasing cable length has a deleterious effect on system throughput. Increases in coax length cause increases in the size of the network collision window--the time during which attempted transmissions by two or more using units can collide--and thus again degrade system performance. And the increased size of the collision window adversely affects the efficiency of data transmissions shorter in time than the collision window.
It is therefore desirable in such communication networks to keep the communication medium as short as possible. But this constraint imposes serious limitations on the extent and configuration, and hence the utility, of such communication networks.